1. Field of the Invention
The invention relates to medical prostheses for implantation in a body, and more particularly, to prostheses which are intended to be fixed into the skeletal structures, such as dental implants and hip joint prostheses.
2. Description of the Prior Art
Medical implants such as dental implants and joint prostheses are normally implanted into the skeletal structure of humans to replace missing or damaged skeletal parts. Often, it is intended that these implants become a permanent part of the skeletal structure. It is important, therefore, that the metal implant be strongly fixed to the skeletal bone structure. Traditional problems, in implant placement and post-operative instability, have been associated with inability to create a precision site, bone die-back associated with the trauma of high-speed burning or over-zealous removal of bone in site preparation, inability of the patient's body to build new bone quickly (as a result of, for example, metabolic bone diseases), physiological barrier to exact implant placement and errors.
Attempts to address these problems have included the use of bone cement for total hip replacement. See, for example, J. Charnley, et al, "The long term reaction of bone to self-curing acrylic cement" J. Bone Joint Surg., 503 822-829 (1968). In cemented arthroplasties, the cement provides initial rigid fixation and stability. However, immediate loading does not allow the prepared site the time to recover from trauma. Furthermore, cement has the complicating factor of additional chance for thermally induced necrosis as curing exotherm heat is released. A fibrous tissue layer is also commonly observed at the interface between bone and acrylic cement. Ideally, however, total bony interfaces are most desirable since a device so fixed would be able to transfer loads efficiently.
In recent years, the search for better mechanisms to increase initial stability and to minimize the occurrence of gradual loosening of prostheses has led to the development of cementless prostheses. The development of cementless prostheses, however, has led to an entirely new set of problems regarding initial stability. Cementless prostheses, must achieve their initial stability by obtaining an exact surgical press fit. Although a large number of prosthesis sizes are available, there will be areas of good contact and other areas of poor contact or gaps between the implant and the bone. This problem may be even greater in patients with osteopenic bone and at cementless revision of failed arthroplasties, which may have large bone defects.
The cementless fixation of porous-coated implants, by bony ingrowth, is also widely used, but this technique is not without associated problems. Recent investigations have shown that for bone to grow into the porous-coated implants, there must be a relatively good mating of the contact surfaces and some form of stabilization and/or patient immobilization during the initial growth phase as to prevent gross movements of the implant. Attempts have been made in the past to shorten the time required for initial implant stabilization by coating porous surfaces with calcium phosphate ceramics. However, such coatings are mechanically unstable and may dissolve under physiological conditions and, thus, the implant may become loose over a long time period. On the other hand, a good mating between implant and bone is still required in order to achieve desirable fixation.